speed at peak torque ntq/(r • min–1)              1500

Max。 cylinder pressure pmax/MPa                   13。5

3。1   Effect of working conditions on the oil 

consumption 

Keeping  the  original  engine  configuration,  five  test conditions  800 (r • min–1), 1 200 (r •min–1), 1 500( r • min–1), 1 800(r  。 m-1),  and 2200 (r • min–1)  are  selected  as  the full-load  performance  of  the  engine  for  simulation。  The required boundary conditions, including the gas pressure in cylinder,  temperature,  and  heat  transfer  coefficient  are calculated。 The gas pressure is measured, and GT-POWER 

is used to simulate the engine working condition and obtain the  gas  pressure  which  is  compared  with  the  measured values for modifying the model。 Then, the gas temperature 

and  heat  transfer  coefficient  of  the  cylinder  are  calculated by  using  the  modified  model  (shown  in  Fig。  6)。  The calculated results of oil consumption are shown in Fig。 7。 It can  be  seen  that  the  peak  oil  evaporation  rate  on  the cylinder  liner  wall  occurs  around  30°CA  after top  dead center (TDC), where the factors, that affect the evaporation rate  such  as  the  cylinder  pressure  and  temperature,  reach the  maximum。  Meanwhile,  oil  evaporation  from  the  liner increases with the increase of power。 The oil blow through the  end  gap  of  the  top  ring  mainly  happens  during  the inhale and exhaust strokes due to the low cylinder pressure during this period。 When the cylinder pressure is lower than the  pressure  of  the  second  ring  land,  a  gas  “upstream”  is formed, carrying the oil and blowing it through the end gap。 The oil throw-off at the top ring happens at the TDC of theexhaust stroke while the direction of the rings is changed to downward  along  with  the  piston,  and  the  oil  accumulated on the top ring is dumped into the combustion chamber due to  the  inertial  force。  With  the  rotating  speeds  of  the  shaft increasing,  the  speed  and  acceleration  of  the  piston  ring increase,  and  the  amount  of  oil  throw  off  at  the  top  ring increases  consequently。  The  oil  scraped  by  the  piston  top land  occurs  at  the  TDC  of  the  inhale  and  exhaust  strokes, which  is  also  increased  with  the  increase  of  speed  and  the amount of scraped oil, showing a similar mechanism to the 

oil throw-off by the piston rings。 

Fig。  8  shows  the  average  oil  consumption  rate  per  cycle for  the  four  modes  under  different  conditions。  The comparison indicates that the evaporation rate is low at 800 (r  •  min–1)due  to  the  relatively  low  cylinder  pressure  and temperature。  However,  the  “upstream”  gas  is  more  serious at  this  speed,  and  the  oil  blow  through  the  ring  end  gap  is higher  than  the  rest。  The  oil  evaporation  from  the  cylinder liner wall dominates under several other conditions。 The oil consumption  is  less  in  other  three  modes,  and  the  ratio varies with the operating conditions。  The  average  oil  consumption rates  with  the varied  loads of 100%,  75%,  50%,  25% and a constant high-torque speed 1  500( r  •  min–1)  are  shown  in Fig。  9  to  further  investigate the  effect  of  the  cylinder  pressure  and  temperature  on  the various oil consumption modes。 The oil evaporation rate on the  cylinder  liner  wall  increases  with  the  increase  of  the load,  while  the  other  three  oil  consumption  modes  are  not  affected by the load。